The purpose of this study is to determine whether better growth and nutritional status can slow the progression of pulmonary disease in young children with CF. This is a 2 year prospective multi-center study of children 6 to 8 years of age at baseline with CF and pancreatic insufficiency. Children are excluded if they have other medical conditions known to affect growth or nutritional status, or if their pulmonary function is less than 40 percent of predicted for FEV1. Approximately half of the subjects will be enrolled in a nutrition intervention program previously shown to be effective in increasing caloric intake and weight gain in children with CF. The protocol involves an overnight admission at baseline, 12 and 24 months to assess resting and total energy expenditure, growth, body composition, sexual and skeletal maturation, dietary intake and pulmonary function. Visits are also scheduled at 6 and 18 months for growth assessment. These are usually conducted at the home CF Center. There have been no changes to the protocol since it's inception. There have been no adverse events related to the study protocol. However, in one case, it was discovered following admission for the study protocol that the subject was not in a state of usual health as is required by the study protocol, but was in the midst of a pulmonary exacerbation. The illness was not related to any aspect of the study protocol. The subject was treated for the exacerbation at CHOP and subsequently transferred to her home CF Center.